Improve claude.md with dev setup instructions

[gerrod3]

I noticed in our new contributors' PRs their agents are failing to properly run the tests since they aren't using the oci-env. So added some instructions for installing it. Before we accept and merge, I would like those using claude code to test it out and see if it does any funny behavior. I'm going to test this with cursor's agent.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough